Shlomi Fish <shlo...@shlomifish.org> writes:
> [...]
>> >> while ( my $numbline = <DATA> ) {
>> >> chomp $numbline;
>> >> my @numbline = split //, $numbline;
>> >
>> > You're processing the input number line-by-line, but it's one whole number
>> > and the products may span across more than one line. You need to slurp it
>> > and prepare an array of digits.
>>
>> Isn't it more efficient to put all the digits into a string rather than
>> into an array and use substr()?
>>
>
> I believe it should be more space efficient, but perl shouldn't have a problem
> managing an array of a 1,000 digits on most machines.
Yes, I'm just wondering.
It doesn't even depend on how many digits there are because it's
possible to consider only the relevant portion of the digits in each
step.
> See https://duckduckgo.com/?q=premature+optimization+evil+quote&ia=web
> - we should worry about optimising when we need to - not before.
I don't agree that all optimization is evil to begin with. I can not
write a line of code without optimizing first because otherwise, I
wouldn't know what to write. Can you?
Knowing what is more efficient helps me before the first line of a
program is written and with every single line of it. I couldn't do any
overall design without knowing.
Even syntax highlighting, reasonable formatting and reasonable use of
comments is an optimization.
The point is to do the right amount of optimization which allows you to
make appropriate progress towards a working version. Once you have that,
you can optimize more or leave it as is.
Programming is a creative process of optimization.
Since we're looking at a mathematical excersise here, we might use it to
learn something about programming :)
> Using arrays allows us to use convenient operators and functions that
> are less native to strings.
Ok, let me be a smartass:
substr() is only so native to strings as perl does not do implicit
typecasting.
The operator in question is multiplication. Are there different
multiplication operators in perl which are more or less convenient?
Did I use inconvenient operators?
> Also note that using http://perldoc.perl.org/functions/vec.html should allow
> to
> store a digit within 4 bits in a binary buffer, which is even better than
> using
> a string.
Hm, I don't understand this documentation. This function seems to be
designed to mess up things by confusing strings with bytes and could be
considered as a symptom of perl not having variables of fixed types.
Maybe I'd understand from a safe and evident use case for it.
> Some comments about your code:
>
> [...]
>>
>> my $data = "
>> 73167176531330624919225119674426574742355349194934
>> 96983520312774506326239578318016984801869478851843
> [...]
>> ";
>>
>
> Please use here-documents for multi-line strings - see
> http://perl-begin.org/tutorials/bad-elements/#string-notation .
Yes, I know. Only I always can't remember how to write those, and I
don't see much of an advantage.
This isn't supposed to be a multi-line string. If a string is one, it
can be written nicely enough, and if it isn't, it's a either a long line
or something that needs to be converted from multiple lines into one.
Is there a way to make sure that a single-line string written in
multiple lines is converted at compile time?
>> $data =~ s/\n//g;
>>
>
> Nice! Perhaps use \s or [^0-9] instead though.
That would have been better. I guess I was too tired to think of it.
>> my $adjacency = 13;
>> my $maxprod = 0;
>>
>> my $start = 0;
>> my $end = length($data) - $adjacency;
>>
>> while($start < $end) {
>
> This should be a «for my $idx ($start .. $end-1)» loop.
Why?
I would think using '$end - 1' is particularly bad because it needs to
be computed for each run of the loop unless the optimizer does a good
job, which I don't know. Since I don't know and since I know that it
can't be optimized out in all cases, I always prefer to use loop
boundaries that do not need to be recalculated for each loop. --- This
is not specific to perl, and while the perl optimizer may do a good job,
optimizers for other languages might not, so generally don't use loop
boundaries that may be calculated for each loop unless it can't be
avoided.
I am --- perhaps overly --- fond of using while() for loops rather than
for(). I'm finding while() loops easier to read because they are more
explicit and easier to use than for() because the notation of for()
loops in different languages varies while the notation of while() loops
does not. You also can easily turn them around by using 'do { ... }
while();' when needed, which you can't do with for() loops.
So what advantage would a for() loop have, in general and particularly
in this case? I would not use '$end - 1', so the for() would require
another variable or some other kind of further ado.
The adornments of for() loops make them intricate; while() loops are
unadorned, hence remain plain and simple, and it is good to write stuff
explicitly as long as it serves to make clear what you mean. A for()
loop doesn't help me making clear what I mean because it is intricate by
its nature.
>> my $adjunct = 1;
>> my $prod = substr($data, $start, 1);
>> print "$prod";
>> while($adjunct < $adjacency) {
>
> Again.
no :)
>> my $ad = substr($data, $start + $adjunct, 1);
>> print " * $ad";
>> $prod *= $ad;
>> $adjunct++;
>> }
>> print " = $prod\n";
>> $maxprod = $prod if($maxprod < $prod);
>>
>> $start++;
>> }
>>
>> print "The largest product of $adjacency adjacent numbers is $maxprod.\n";
>>
>> exit 0;
>>
>
> No need for the exit statement here.
Write stuff explicitly and tidy. The program ends here, even defining a
return code. You can easily see that right away.
Otherwise, I would need to enable the scroll bars and look at them, or
move point to the end of the buffer, or do something else to be sure.
It's not needed but clear. I do like adornments when they're helpful.
>>
>> Is this supposed to be a programming exercise? It's a purely
>> mathematical one to me.
>>
>
> See https://en.wikipedia.org/wiki/Project_Euler for the Project Euler mission
> and motivation.
Why is it that everyone interested in programming is assumed to be
interested in mathematics?
I can't do mathematics at all because I understand things by removing
abstractions. Remove all abstractions from mathematics and nothing
remains. I can only use it, to some limited extend, like a computer
executes a program. That isn't very interesting, and I have computers
to do that for me so I don't need to.
--
"Didn't work" is an error.
--
To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
